Vibration tables are used primarily in connection with test fixtures designed to test products under vibrating conditions. The general construction of vibration tables is well known. Generally they include upper and lower plates separated by spacers, with a vibrator mounted on the bottom of the lower plate and a test product mounted on the top of the upper plate. Representative vibration tables are shown in Applicant's U.S. Pat. No. 5,594,177 (which is incorporated by reference) and U.S. Pat. No. 5,402,991.
An object in the design of a vibration table is to provide a table that transmits a high proportion of the vibration energy to the test product over a desired frequency range. In order to accomplish these goals, it is generally desirable to make the table as rigid as possible and yet as light as possible. Rigidity maximizes higher frequency and higher energy transmission and minimizes resonant frequencies at low frequency ranges, while a light weight table minimizes energy loss that results from the mass of a heavy table. To accomplish these goals, tables are frequently made with a hollow or semi-hollow core. In Applicant's U.S. Pat. No. 5,594,177, the table includes spaced upper and lower plates connected together at the periphery by edge spacers and having intermediate spacers interconnecting the plates at intermediate positions.
Another feature of vibration tables is that they are frequently used to test products in high and low temperature environments. Thus, an insulating sheet is often bonded to the top of the upper plate. A metal sheet formed of titanium or the like can be mounted on top of the insulation. A problem with the incorporation of insulation in a shaker table is that the insulation can have a dampening effect on vibration that reduces the energy transmitted by the vibration table and the frequency response of the table. Prior tables typically have been limited to transmission of forces of under about 70 Gs RMS, with the great majority of the force being transmitted in a frequency range of about 1000-3000 Hz. This issue was addressed by Applicant's U.S. Pat. No. 7,886,606, incorporated herein by reference.
A further issue that affects the use of vibration tables, and the consistency of results obtained, lies in the variability of vibrations from point to point over the surface of the table. In a table having a conventional square or rectangular upper surface, the vibration profile (i.e., the amplitude per frequency of the vibrations over a frequency range in each of the x-, y-, and z-axes) can vary up to 20% over the surface of the table (i.e., at various points in the x-y axis) due to resonance of the vibrations reflected and concentrated by the sides and corners of the upper surface.
An object of the present invention is to provide a lightweight table that has improved energy responsiveness and a broader frequency spectrum and provides a means for uniform vibration profiles over the surface of the table. Specifically, it is an object of the present invention to provide a vibration table demonstrating a vibration profile over the surface of the table having a variability of less than 5%, and preferably +/−3%.